Technology advancement in wireless and mobile communications in recent years has resulted in a wide spread deployment of various mobile and wireless networks, including GSM, CDMA, satellite, WLAN, WiMAX and Bluetooth, etc. Each network is tailored to suit particular market trends, or particular types of users with special service needs. Such a network diversity offers different choices in terms of bandwidth, security, and coverage areas for mobile users. For example, WLANs offer high bandwidth with limited mobility scope, whereas cellular networks offer universal network access with lower bandwidth and higher costs. Even within the same type of networks, conditions of different cells or subnets change dynamically. Considering all these differences, mobile terminals or networks have to face problems such as how to select the best network, when to initiate a handover, how to achieve seamless handovers between networks. Moreover, with the development of the multimedia communication, many new types of network applications come up with different requirements on network services. Hence it is important to apply an appropriate handover decision algorithm to satisfy different application's requirements under different network conditions/configurations.
Various handover decision algorithms have been proposed in past few years, and all the work concentrates on the algorithms with RSS as the main criterion. Traditional handover decision algorithms rely on Received Signal Strength (RSS) and a handover is initiated when the RSS is below a specific threshold. But RSS only indicates the usability of a network, and cannot give more details on a network such as the available bandwidth which is more meaningful for upper-layer applications. For a terminal handover between heterogeneous networks, RSS is also not suitable for being a criterion due to the different physical technologies adopted by different networks. In addition, for traditional RSS-based handover decision algorithms, a channel model is adopted to evaluate an RSS-criterion-based handover algorithm, in which such three performance parameters as a lost call probability, an unnecessary handover probability and a handover probability are defined.
To better meet the requirements of upper-layer applications, link-layer parameters (such as bandwidth, delay) are also considered, especially for terminal handover across heterogeneous networks (so-called vertical handover). Along with the proposal of such a lot of algorithms, it needs to discuss how to evaluate these handover algorithms. All these algorithms are proposed in an intuitive way without any guarantee of the wrong decision probability (WDP).